Neutralizing circuit for screen-grid tubes



'May 31, 1938. w, BUSCHBECK 2,119,315

' NEUTRALIZING CIRCUIT FOR SCREEN GRID TUBES Filed April 3, 1935 2 Sheets-Sheet 1 ourpurw INPUT INVENTOR WERNER BUSCHBECK ATTORNEY M y 31, 1938. w. BUSCHBECK 2,119,315

NEUTRALIZING CIRCUIT FOR SCREEN GRID TUBES Filed Alzfil 3, 1955 2 Sheets-Sheet 2 INVENTOR WERNER BUSCHBECK BY WM ATTORNEY Patented May 31, 1938 UNITED STATES NEUTRALIZIN CIRCUIT FOR SCREEN-GRID TUBES Werner Buschbeck, Berlin, Germany, assignor to Telefunken Gesellschaft fiir Drahtlose Tele-' graphie m. b. H., Berlin, Germany, a corporation of Germany Application April 3,

1935, Serial No. 14,494

In Germany April 3, 1934 4 Claims.

The present invention is concerned with ways and means adapted to neutralize or compensate residual capacitances existing in screen-grid tubes between the control electrode and the 5 plate, and which are especially suited for short- Wave. systems.

Owing to the presence of the screen-grid the capacitance existing between plate and control electrode is effectively diminished. The residual capacitance which may remain is neutralizable with such means as are known from triode tubes as long as this assumption is fulfilled; that the screen-grid so far as alternating currentvoltage is concerned is at cathode potential. And this assumption may be supposed'tb be practically satisfied where not unduly high tube capacities are dealt with, where the waves are long, and where an adequately high blocking condenser is provided between the screen grid and the cathode. Where however shorter waves are concerned, and where screen-grid tubes are designed for large powers,'the said assumption is no longer satisfied. Of necessity, the connecting leads between the electrodes and the leadin points or seals across the vacuum vessel (with the exception of the plates in water-cooled tubes) are of a certain length and thus have an inductive reactance which," where short-waves are involved, is no longer negligibly small in comparison with the capacitive reactance residing in the correspondingintermediate-electrode ca-- pacitances. Even if the supposition were made that the capacity interposed between the screengrid and the cathode were free from loss (leakage) and infinitelyhigh, the screen grid can no longer be considered, as regards high-frequency, to be at cathode potential. However, this means that in the case of the arising of a plate alternating voltage, there is impressed upon the screen-grid an alternating voltage of the same frequency, but a phase displacement angle of 180 degrees, the amplitude of which is' governed by the voltage division due to the capacitance (Cs'A) prevailing between the screen-grid and the plate, and the lead inductance (Ls) existing between the screen-grid and the cathode. In other words, the screen-grid, so iaras the control electrodeis concerned; acts like a further anode or plate; which, to be sure, is subject to alternating voltages" of lower amplitude, but which because ofthe considerable capacitybetween the screen-gridand the control electrode (Cos) is in rather close coupling relation with the latter. In view of the fact that the alternating voltages at the plate and the screen-grid present a relative phase angle of 180 degrees,

it ispossible by choosing suitable dimensions or values for the inductance existing between the screen-grid and the cathode (Ls) to secure perfect neutralization for a very definite wavelength. However, such neutralization is inadequate in practical working inasmuch as it would be lost and lacking as soon as there is a change in the wave-length, while the restoring of the state of neutralization would be attended with a good deal of tedious and time-consuming readjustment in every given instance. The present invention, therefore, discloses ways and means incorporated in a circuit organization whereby neutralization of the residual capacitance between control grid and plate will be insured under conditions unaffected by the wave-length or frequency.

According to the invention, by the simulation of all of the capacitances existing inside the tube,

tween plate and cathode, or between screen-grid and cathode, which anyway are always inherently small.

A number of exemplified embodiments of the basic idea of the invention are shown in the accompanying drawings, wherein Fig. 1A is a 30 screen-grid circuit neutralizing arrangement according to the invention; Fig. 1B is a circuit equivalent, to that shown in Fig. 1A, but redrawn in a different form; Fig. 2 is a similar form of circuit as shown in Fig. 1B utilizing tubes in 1 push-pull according to the invention; Fig. 3A is a modification of the arrangement shown in Fig. 1A, but employing grid-circuit neutralization, and Fig. 3B is an equivalent and redrawn circuit.

Referring in particular to Fig. 1A, a stage of amplification is shown as comprising a screengri-d tube V whose cathode is denoted by K, the control grid by G, the screen-grid by S,and the plate or anode by A. The plate potential, by ways and means known in the art, is supplied by the aid of a radio frequency choke coil D, while the plate oscillation circuit LCC is relieved of direct voltage by the provision of a blocking capacity B. The radio frequency control voltage is impressed upon the grid circuit across the terminals I] and I. The screen-grid potential is furnished from a source of voltage Else. The inductances of the'leads or wires brought to the various electrodes, in whichare figured also the leads disposed inside of the vacuum vessel, are indicated in the form of lumped inductances Lo, Ls and LK. According to this invention, neutralization under conditions independent of frequency or wave-length is secured by the fact that all of the capacitances existing within the tube as well as the inductances of the leads brought to the different electrodes are simulated; in fact, such simulation, as it were, constitutes a homologous complement of the tube V including all of the leads and wires.

The neutralizing action of Fig. 1A may be more readily understood by reference to Fig. 1B. The object of the invention is to arrange the circuit in such a way that regardless of frequency, voltage between output terminals will not produce any voltage between input terminals. This result can be achieved by making the entire network symmetrical in the sense that each of the input terminals has a potential which is intermediate between the potentials of the output terminals in the same proportion. While not the only way to obtain this result, the simplest way is to duplicate all the impedancesof the tube system (shown in the upper half of the network of Fig. 1B) by a similar set of impedances (shown in the lower half of Fig. 1B) and which may be called the neutralizing network. In this way it is obvious that the potentials of the input terminals are each half way between the potentials of the output terminals and since the input terminals have the same potential (in the absence of impressed signal voltages) in spite of voltage across the output, the object of the invention is attained.

The inductances serving to simulate the inductances associated with the tube are characterized by a supplemental index N (thus: LNG, and LNs), whereas the simulations in regard to capacitance are indicated and designated by NGK (grid-cathode capacitance), NGA (grid-plate capacitance), NsG (screen-grid/control-grid capacitance), and NSA (screen-grid/plate capacitance). Condenser NKA (cathode-plate capacitance) and NSK (screen-grid/cathode capacitance) may be omitted, under certain circumstances, provided that inherently low' capacities are involved, and that thewaves are not unduly or abnormally short. 7

Fig. 113 represents merely the same circuit organization drawn in a different form, with the tube being imagined to be resolved into its constituent elements. The reference numerals are the same as in Fig. 1A, and the symbols used for the different electrodes of the tube are in the same form as shown in Fig. 1A.

For the balance and simulation of the capacitances and inductances arising inside the tube, the use of an inactive tube similar to the tube to be neutralized is particularly well suited. In this respect the several neutralizing capacitances and inductances shown in Figs. IA and 1B may be considered as corresponding to the interelectrode capacitances and lead inductances of the inactive tube. According to the prior art, such tube phantoms have merely been employed for the purpose of allowing of line (circuit) reduction in transmitter stages containing more than one tube by that a working tube was replaced by a phantom corresponding to it. However, according to this invention such a simulation is utilized for a different purpose and in a difierent sense, i. e., with the end in view to securing neutralization in an amplifier valve under conditions independent of the frequency.

Inasmuch as the simulation of a capacity, under certain circumstances, is more readily feasible than that of an inductance, it may if conditions are appropriate, be feasible to interpose in the leads of the alternating potential applied to the different electrodes, condensers, and to render the resultant impedance (apparent resistance) either ohmic or capacitive.

Fig. 2 shows an extension of the arrangement shown in Fig. 1B for a single tube, as applied to a push-pull arrangement. The reference symbols referring to the second tube are characterized by primes It will be unnecessary to give further explanations for this circuit diagram.

It is known that the voltages required for such compensation or balancing may be derived either from the grid or from the plate circuit of the stage'in question. Whereas in the case of Figs. 1A and 1B neutralization was effected from the plate end, Figs. 3A and 3B illustrate a circuit organization in which neutralization, in a way as known basically from the prior art, is efiected from the grid. The input circuit of the stage contains the inductance LI as well as the condensers Cl; the output circuit contains the inductance L and shunt condenser C; the other circuit elements are the same as those used in the preceding figures. representation of the circuit scheme illustrated in Fig. 3A in order that the homologous or mirror-picture simulation may be made more clearly understandable.

What I claim is:

1. A circuit arrangement comprising a tube having a cathode, a control grid, a screen grid electrode within said tube intended to operate at unvarying potential but actually fluctuating in potential when operated at very high frequencies due to unavoidable or slightimpedances to high frequencies in the circuit between said screen grid electrode and cathode, input and output circuits associated with said tube, and means for counteracting energy transfer from output to input circuits resulting from said fluctuating potential, said means comprising a pair of series-connected inductances having their common terminals connected to the tube cathode, an inductance connected to the control grid, the opposite end of said last mentioned inductance being connected to the unconnected terminal of one of the pair of inductances by means of a condenser, said opposite end of said inductance being also connected to the unconnected terminal of the other inductance of the pair by means of a pair of series-connected condensers.

2. A neutralizing circuit'for an electron discharge device provided with anode, cathode, control grid and screen grid electrodes, comprising a first inductance of a value substantially equal to the self-inductance of the cathode lead having one end connected to the cathode, a first capacity of a value substantially equal to the grid-cathode capacity having one end connected to the other end of said first inductance, a second inductance of a value substantially equal to the self-inductance of the screen grid lead having one end connected to the cathode, a second capacity of a value substantially equal to the screen grid-anode capacity having one end connected to the other end of 'said second inductance, a third inductance of a value substantially equal to the self-inductance of the control grid lead having one end connected to the control grid and the other end connected to the other end of the first mentioned capacity, and a third capacity of a value substantially equal to the control gridanode capacity connected between the last mentioned connection and the other end of said second capacity.

3. The circuit according to the invention defined in the preceding claim wherein a capacity of a value substantially equal to the screen gridcontrcl grid capacity is connected between the non-common terminals of the second and third. capacities.

4:. A neutralizing circuit of bridge form for an electron discharge device provided with anode, cathode, control grid and screen grid electrodes, comprising a condenser in the output circuit of said device connected between said anode and cathode, a first inductance of a value substantially equal to the self-inductance of the control grid lead, a second inductance of a value substantially equal to the self-inductance of the screen grid lead, a direct connection from one terminal of said first inductance to said control grid lead, a direct connection from one terminal of said second inductance to said cathode, the other terminals of said two inductances being connected together through a condenser whose value is equal to the value of the internal capacity between said control grid and screen grid electrodes, two serially connected condensers arranged in parallel with respect to said last-named condenser, one of said two serially arranged condensers having a value substantially equal to the value of the internal capacity between said anode and control grid, the other of said two condensers having a value substantially equal to the internal capacity between said anode and screen grid, a connection between the junction point of said two condensers and said output circuit, and an inductance and a condenser connected in series between the cathode and said other terminal of the first inductance, said inductance being of a value substantially equal to the self-inductance oi the cathode lead and said condenser being of a value substantially equal to the value of the internal capacity between said control grid and cathode electrodes.

WERNER BUSCHBECK. 

